So, yes, the initial fault is United's. But there's blame enough to go around.
If I were in his position I'd quietly fume about it and United would get more comfortable about booting people who have boarded without repercussions. It's pretty rare that passengers' cases get brought to light so it's kind of refreshing.
If he had just left when the police arrived, as commenters above you are suggesting, then that would mean that the threat of violence was a successful solution to United's mistake, and we'd never have heard about it.
Otherwise this becomes might makes right.
Note well: I am not saying that the cops are always right. I am not saying that we should all be good little citizens and obey whatever the authority figures tell us to do. I am saying that, in the actual world we live in, disobeying police orders will often involve painful physical consequences, almost immediately. You can argue about whether that should be. Fine. I'm saying that it is that way, though, and you'd better plan on it being that way in your situation before you decide to ignore what the cops tell you to do.
> Because the cops told him to, that's why.
"I am not saying obey authority figures whenever they tell us to do something. Even though I said in the previous sentence, 'because the cops told him to, that's why'."
Your note completely and utterly contradicts your previous statement. There is no way to reconcile them.
That is, when the authority figures are morally in the wrong, you can make a case that the moral thing to do is to disobey them. But just in terms of the actual way things work in this less than perfect world, you'd better think seriously about the price you're going to pay for doing so. You can say "they shouldn't use force". Fine. But in the real world, they're going to use force.
So, no, I'm not contradicting myself. I'm speaking in two different senses. There's the ideal world, and there's the practical way things actually work. In the real world, if you disobey an order from the cops, it's going to hurt you, no matter how morally right you are.
Trying to solve business disputes by force, which is what the passenger did by refusing to leaves, is bad policy.
It's amazing how Americans can justify the most extreme violence for the most petty of reasons. This incident was absolutely meaningless _until_ this man had his face bloodied.
There were a thousand different options, besides force. Do that instead!